Image sizes are shown directly in the file list panel. The size is shown in the column named "Extra". You can drag and drop to change the column order and also set the column visibility to suit your needs.
Most of the widely supported image formats are supported: PNG, JPEG, JPEG-2000, TIFF, PICT, GIF, Quick Time Image, AppleICNS, BMP, ICO, Raw Image, Scalable Vector Graphics and Live Photo.